GA pilot reported while in cruise flight encountering mountain wave conditions resulting in loss of engine performance and loss of altitude. The GA pilot diverted to the closest airport.

Narrative

I was on an IFR flight plan from ZZZ1 to ZZZ2 when about 25 miles East from ZZZ along our route I suddenly experienced a loss in engine performance (loss of 50 miles in airspeed); I was unable to keep altitude with the performance loss and had to pitch nose down to not get into a stall situation. Tried to identify the engine issue and then decided to divert to ZZZ to get the plane safe on the ground for service. Did not immediately contact ATC when the problem occurred to focus on flying the airplane and identify the problem. Lost about 400 feet when ATC called to ask to check the altitude. Reported the problem and when unable to find a solution still losing altitude and deviated to ZZZ. Landed the plane safely.I flew the plane first and should have called ATC immediately when losing altitude and also should have considered mountain waves as a potential cause for the issue.Plane will get checked thoroughly by service and only be taken back up if signed off by the authorized service technician. Logbook entry will be required and also mountain wave avoidance training is booked with a certified instructor.